1) Preheat the oven to 350°F. Lightly grease a 9" pie pan.
2) To make the crust: Whip together the egg whites and cream of tartar until foamy.
3) Gradually add the sugar, whipping until stiff peaks form.
4) Spread the meringue into the bottom and up the sides of the pan; it can overflow the edge slightly.
5) Bake the crust for 30 minutes, until the surface is dry and slightly golden. Turn the oven off and leave the crust in the oven for 2 to 3 hours, until almost cool; it shouldn't be completely dry.
6) To make the filling: Bring the water to a boil in a saucepan set over medium heat. Add the pastry filling mix and whisk vigorously to blend as the mixture comes back to a boil.
7) Remove the pan from the heat, and whisk in the butter, stirring until it's completely melted.
8) Transfer the filling to a bowl and chill it in the refrigerator for a couple of hours.
9) Whip the heavy cream until stiff peaks form. Fold the cream into the cooled filling until thoroughly incorporated.
10) Spoon the filling into the cooled crust. Refrigerate the finished pie for 1 to 2 hours, until set. Garnish with lemon slices and/or whipped cream, if desired.
Yield: about 8 to 10 servings.